Composite materials have the capability to deliver increased structural performance over traditional metal or polymer options. Typically, composites combine the environmental resiliency of polymers with the strength and stiffness properties of the fibers from which they are derived. This family of fibers most regularly includes glass, aramids, and carbon. Composite fibers can be woven into fabrics, which are typically classified by type, e.g., satin weave, twill, burlap, etc., using the same nomenclature as other textiles. Composite fibers may also be gathered in bundles.
In a fiber composite material, the fiber properties drive the primary material properties. Fiber properties are extremely orthotropic, meaning they have dramatically different properties in the fiber and cross-fiber directions. These highly orthotropic properties are both the advantage and disadvantage of composite materials. Composite structures can be designed with structural material placed only in the areas required, and the properties of the material can be tailored to the specific area and type of stress they see in operation. A simple example is that of an “I” beam, where straight fibers are used along the caps and those fibers see primarily tension and compression loads, while biased fibers are placed on the beam web where primarily shear loads are present.
When this highly tailored, orthotropic design approach is used significant weight savings can be realized over traditional homogeneous material properties. However, the fiber laminate material properties are susceptible to slight changes in fiber orientation during manufacturing. For instance, a design may call for a 45-degree orientation for optimal performance, and shear stiffness performance will generally change with the sine of the angle, meaning a 5-degree misalignment will reduce axial properties by as much as 8.7% off the peak axial value. A variation of 5 degrees is not uncommon after manual handling of fabric and application using large manual tools.
The higher the degree of orthotropy in the fiber, the more susceptible that a lamination made from that fiber is to misalignment. For example, fiberglass unidirectional tape has extensional stiffness of approximately 5.6×106 lb/in2 and lateral stiffness of 1.3×106 lb/in2, a ratio of 4:1. In contrast, carbon fiber has an extensional stiffness of 16.7×106 lb/in2 and lateral stiffness of 1.7×106 lb/in2, a ratio of 10:1. Therefore the fall off of axial direction properties is much more sensitive with carbon unidirectional fibers than it is for glass unidirectional fibers.
In the creation of large composite structures, for example wind turbine blades, vehicle chassis, and structures for building construction, much of the fiber and/or fabric placement is conducted by hand, using manual tools. Fibers and/or fabrics are placed into an infusion tool dry, which allows for considerable slippage during the infusion phase. Dry fibers and/or fabrics must be used because of the limited handling time of mixed resins. Typically, resin is infused into the structure after dry fiber and/or fabric placement. Additionally, dry fibers and/or fabrics are traditionally handled and positioned together in an infusion tool at the final manufacturing site because of the difficulty in controlling their relative positioning. Pre-positioning of fabrics, followed by movement of the mold or transportation to an infusion site would cause significant misalignment and movement of dry fibers.
The majority of large composite structures are laminated at a location close to the place where the structures will be manufactured due to the limited handling time and reduced workability of a pre-infused or “prepreg” fibers and/or fabric. Although fiberglass infusion has been well-proven in large structures, as the need for even larger structures increases, the use of stiffer additive materials will be necessary. Unfortunately carbon fiber is not as absorptive as fiberglass, and attempts to infuse structures made of a combination of fiberglass and carbon often create voids and dry regions in the final structure. One method of increasing the wetting ability of carbon fiber in a laminate is to intersperse it within a fiberglass laminate. This allows resin to infiltrate thinner stacks of carbon layers and has the added benefit of dispersing the high tension loads of a carbon layer into the adjacent fiberglass structure through interlaminar shear.
Interleaving different materials introduces increased labor into the already time-consuming process of placing the composite materials into an infusion tool. For large structures the process of placing fibers and/or fabrics in the infusion tool is a major cycle time restriction. To alleviate this, large structures are often made of thicker fibers and/or fabrics to enable a rapid buildup of material thickness in the infusion tool. In addition, weaves of several fabric directions have been created to allow placement of several different plies at once, although these multi-axial fabrics are expensive. In some cases, mixes of carbon and glass fibers have been created. However all these methods are limited in total single ply laydown thickness to around 0.080 inches. This means that a large structural laminate with a thickness of 1-1.5 inches will require a minimum of 12-18 plies.
There is, therefore, an unmet need to have the ability to accurately control the positioning of dry laminate plies, particularly for relative fiber orientations, in an infusion tool.